A Swiftly Tilting Planet (Time Quintet #3) by Madeleine L'Engle

In this companion volume to A Wrinkle In Time (Newbery Award winner) and A Wind In The Door fifteen-year-old Charles Wallace and the unicorn Gaudior undertake a perilous journey through time in a desperate attempt to stop the destruction of the world by the mad dictator Madog Branzillo. They are not alone in their quest. Charles Wallace's sister, Meg - grown and expecting her first child, but still able to enter her brother's thoughts and emotions by "kything" - goes with him in spirit.